The study shows that a thymus‑derived peptide called thymalin can boost the growth of certain blood‑forming cells (granulocytic colonies) in the spleen of mice that had their thymus removed, and it can also help normalize bone‑marrow cell development when given to live animals. This effect seems to involve a special T‑cell that directs how these colonies form, and the peptide works both in test‑tube (in vitro) and live‑animal (in vivo) settings.

It has been previously demonstrated by the authors that histological characteristics of colony-forming units (CFUs) in normal mice prove a certain shift in their differentiation in erythroid direction comparing to the bone marrow CFUs. Thymectomy of mature animals is accompanied with weakening growth of granular colonies at cloning of the bone marrow CFUs and with loss of stability in direction of splenic CFUs differentiation. Polypeptide preparation of the thymus--thymalin stimulates growth of the granulocytic colonies from the splenic CFUs in thymectomized mice both in in vivo and in vitro experiments. Differentiation of the bone marrow CFUs is normalized under the effect of thymalin in in vivo experiment only. The data obtained confirm the suggestion made by R. V. Petrov on existence of T-cell clone, enhancing CFUs differentiation in granulocytic direction. Activation of this clone in the spleen is revealed at thymectomy and stimulation of the cells with thymalin both in in vivo and in vitro experiments. Thus, affirmations are obtained on differences of clonic T-cell regulation of the CFUs differentiation in the bone marrow and in the spleen.